It is well known that disperse solid materials have the undesirable tendency of caking or aggregating during storage and/or transportation. Furthermore, it is well known that the tendency of caking is dependent on a number of factors, for example, ambient conditions, the shape of a container, and the like.
Conventional methods of evaluating the stability of disperse solid materials against caking can be divided into two approaches. In one approach, a disperse solid material is evaluated by measuring the smallest compression load required to aggregate a sample of material. In another approach, a disperse solid material is evaluated by measuring a particular mechanical characteristic of an aggregate formed as a result of the compression of a sample of material by a known compression load.
One method applying the first approach is known as the Newton method and involves applying a compression load to a sample of a disperse solid material and concurrently measuring a decrease in the volume of the sample. The stability of the material against caking is deduced from the compression load at which no further decrease in volume occurs. The method is described in an article entitled "A Method of evaluating the mechanical characteristics of powder from the determination of the strength of compacts" by Newton G. M. et al., Powder Technology, 1992, Vol. 72, No. 1, pp. 97-99.
Generally speaking, the methods applying the first approach suffer from the disadvantage that the transition from a disperse state to an aggregate state is a gradual process and therefore there exists considerable discrepancy as i to the "degree of caking", and the corresponding compression load which achieved that degree of caking, which is sufficient for a sample to be regarded as having been aggregated.
One method applying the second approach is known as the Jenike method and involves determining the relation between the compression load employed to aggregate a sample and the shear stress required to shear the aggregate. The Jenike method is described in a book entitled "Principles of Powder Technology", John Wiley & Sons, N.Y., 1990, pp. 91-118.
Another method applying the second approach is described by Andrianov, Certificate of Authorship of the USSR, No. 752153, Inventions and Discoveries, 1980, No. 28 and involves placing a sample of a solid disperse material in a cylindrical :chamber, applying a pre-determined pressure, typically 50 kPa, to the sample, maintaining the sample at a temperature of 350.degree. C. and measuring the tear stress of the resulting aggregate.
Generally speaking, the methods applying the second approach suffer from the disadvantage that a mechanical characteristic of an aggregate is not a metric of the stability against caking of a disperse solid material but rather a metric of the aggregate itself.
It is also well known that solutions to overcome the tendency of caking include selection of the particle size of the disperse material, adding anti-caking agents, and the like.
The method most widely used for selection and comparison of anti-caking agents includes applying compression loads to samples of material mixed with the same proportion by mass of different anti-caking agents. The efficiency of each anti-caking agent can be deduced from the compression load required to cause caking, namely, the efficiency of the anti-caking agent is higher if a greater compression load is required to aggregate a sample. This method applies the first approach described hereinabove and therefore suffers from the same disadvantage.